Au-rich Porphyry Research Articles

District-Scale VMS to Porphyry-Epithermal Transitions in Subduction to Postcollisional Tectonic Environments: The Artvin Au-Cu District and the Hod Gold Corridor, Eastern Pontides Belt, Turkey

Abstract Porphyry, epithermal, and volcanogenic massive sulfide (VMS) deposits can form together in the same mineral district in convergent margin environments. Their spatial association and superposition indicate evolving tectonic settings. The Artvin Au-Cu district is one of the major clusters of VMS bimodal-felsic, porphyry, and epithermal deposits in the Eastern Pontides belt in northeast Turkey. Whereas ore-forming processes, timing, and tectonic setting of VMS mineralization are well defined in Artvin, those for porphyry and epithermal mineralization remain less constrained. Our district-scale field study focused on the Hod gold corridor in the Artvin district, which is defined by the NE-trending alignment of the recent Au-Cu mineral discoveries (~205 t Au; ~0.33 Mt Cu; e.g., Hod Maden, Ardala-Salinbaş, and Taç-Çorak) that include Au-rich porphyry, highand intermediate-sulfidation epithermal, carbonate-replacement, and hybrid VMS-epithermal mineralization styles. Our new U-Pb, 40Ar/39Ar, and Re-Os geochronological results interpreted with previously compiled data show that magmatism in the Artvin district formed in the Carboniferous (358–325 Ma), Jurassic (182–174 Ma), Late Cretaceous (92–78 Ma), Eocene (51–40 Ma), and Oligocene (30 Ma). Porphyry and epithermal mineralization along the Hod gold corridor peaked in the Early (~113 Ma; Berta prospect) and Late Cretaceous (~86.5-82 Ma; e.g., Taç and Çorak deposits) and Eocene (~50 Ma; e.g., Ardala deposit), whereas VMS bimodal-felsic mineralization only formed in the Late Cretaceous (~91–85 Ma). Therefore, we interpret that the Hod gold corridor was a long-lived, deep crustal-scale structural feature along which the successive magmatic and mineralization events were emplaced. In addition, the timing of porphyry Cu-Mo mineralization can significantly (>20 Ma) postdate the crystallization age of the intrusive host rocks in the Artvin district, such as at Berta and Balcılı camp, which emphasizes the importance of dating mineralization directly to correctly attribute the tectonic setting. The distribution of Late Cretaceous mineral occurrences suggests a possible eastward temporal evolution from VMS (~91–85 Ma) to epithermal-porphyry systems (~86.5–82 Ma), transitioning from back-arc to arc settings at the onset of the Northern Neotethyan oceanic slab rollback and accompanied by increasingly elevated gold content eastwards across the Artvin district.

Early Au-rich sulfide liquid saturation explains the low Au endowment of continental intraplate alkaline magmas

Abstract Alkaline magmatic systems formed in intraplate settings have a low Au endowment, contrasting with the high Au fertility of alkaline magmatic systems in arc and rift settings, which host numerous Au-rich porphyries and Cu-Au and Au-Ag epithermal deposits. Among other factors, the Au fertility of a magmatic system is determined by the Au concentration of the ore-forming magma, which is strongly controlled by the amount and chemistry of magmatic sulfides that fractionated during magma differentiation. To better explore the factors controlling the low Au endowment of alkaline magmatic systems formed in intraplate settings, we studied magmatic sulfide and silicate melt inclusions hosted in various phenocrysts from the Mont-Dore Massif (France). The magmatic system was saturated with Cu-poor, Au-rich sulfide liquid during its differentiation from basanite to trachyandesite, leading to a strong depletion of both Cu and Au in the residual melt. This presumably reduced the capacity of the magmatic system to form Au-rich magmatic-hydrothermal deposits. Such evolution contrasts with previous studies conducted in sulfide-saturated magmatic systems formed in arc settings, where an Au-poor monosulfide solid solution was the main sulfide phase to crystallize, only marginally affecting the Au budget of those systems. We conclude that the observed saturation of Au-rich sulfide liquid during evolution of the studied volcanic products could explain the low Au endowment of intraplate alkaline magmatic systems.

Role of magma differentiation depth in controlling the Au grade of giant porphyry deposits

Porphyry deposits are the world's most important source of Cu and a major source of Au. It has been recognized that low Au grades generally characterize porphyry deposits in thick continental arcs, like the Andes, where the magmas are likely to differentiate at depth. In contrast, Au-rich porphyries are mainly found in thin island arcs, for example, those of the southwest Pacific, or associated with short extensional periods in continental arcs, where the magmas are likely to experience relatively shallow differentiation. However, the key factors that control this difference remain debated. This study shows that the Au grade of giant porphyry deposits and La/Yb ratios of the ore-associated suites are negatively correlated (r=∼0.7, p=10−7). We attribute the negative correlation to be mainly due to varying sulfide saturation histories, modulated by the depth of magma differentiation.Magmas differentiating in deep crustal reservoirs are likely to reach sulfide saturation early due to high pressure and early depletion of FeO (calc-alkaline trend). Early sulfide saturation causes most Au to be held in cumulus sulfides, making it unavailable to enter ore-forming fluid released in the upper crust, resulting in Au-poor porphyry systems. In contrast, magmas evolving in shallow reservoirs are likely to experience late sulfide saturation because of the high sulfur solubility induced by low pressure and the high FeO content of the melts (tholeiitic trend). Late sulfide saturation enhances the potential of a magmatic system to form Au-rich porphyry deposits due to the high Au content of the magma at voluminous fluid saturation and efficient Au transfer from melt to the ore-forming fluids via sulfide-fluid interaction. The link between average magma differentiation depth and Au content is supported by platinum-group element geochemistry of the porphyry ore-forming suites, which shows that the magmas differentiating at shallower depths (lower La/Yb) reach sulfide saturation later and therefore have higher Au concentrations than those that differentiate at deeper levels (higher La/Yb). Numerical models for ore-associated magmas with different sulfide saturation histories indicate that variations in the Au grades of giant porphyry deposits can be explained by the variations in the timing of sulfide saturation. Based on the results, we propose that Au concentration in the magma, modulated by average magma differentiation depth and sulfide saturation history, is one of the critical factors controlling the Au grade of giant porphyry Cu deposits.

Machine learning for geochemical exploration: classifying metallogenic fertility in arc magmas and insights into porphyry copper deposit formation

A current mineral exploration focus is the development of tools to identify magmatic districts predisposed to host porphyry copper deposits. In this paper, we train and test four, common, supervised machine learning algorithms: logistic regression, support vector machines, artificial neural networks (ANN) and Random Forest to classify metallogenic ‘fertility’ in arc magmas based on whole-rock geochemistry. We outline pre-processing steps that can be used to mitigate against the undesirable characteristics of geochemical data (high multicollinearity, sparsity, missing values, class imbalance and compositional data effects) and therefore produce more meaningful results. We evaluate the classification accuracy of each supervised machine learning technique using a tenfold cross-validation technique and by testing the models on deposits unseen during the training process. This yields 81–83% accuracy for all classifiers, and receiver operating characteristic (ROC) curves have mean area under curve (AUC) scores of 87–89% indicating the probability of ranking a ‘fertile’ rock higher than an ‘unfertile’ rock. By contrast, bivariate classification schemes show much lower performance, demonstrating the value of classifying geochemical data in high dimension space. Principal component analysis suggests that porphyry-fertile magmas fractionate deep in the arc crust, and that calc-alkaline magmas associated with Cu-rich porphyries evolve deeper in the crust than more alkaline magmas linked with Au-rich porphyries. Feature analysis of the machine learning classifiers suggests that the most important parameters associated with fertile magmas are low Mn, high Al, high Sr, high K and listric REE patterns. These signatures further highlight the association of porphyry Cu deposits with hydrous arc magmas that undergo amphibole fractionation in the deep arc crust.

Gold endowments of porphyry deposits controlled by precipitation efficiency

Porphyry deposits are natural suppliers of most copper and significant gold to our society. Whereas the Cu-richest (Au-poor) porphyries are related to Andean-type subduction and typical calc-alkaline magmatism, the Au-richest porphyries are associated with high-K calc-alkaline to alkaline magmatism in late to post-subduction or post-collision and extensional settings, and subordinately with calc-alkaline magmatism. The reasons behind these associations and the large variations in metal endowments of porphyry Cu–Au deposits remain obscure. Here, I show that porphyry Cu–Au deposits define two distinct trends in Au vs. Cu tonnage plots (Cu-rich and Au-rich). Metal endowments for both trends grow larger the longer the mineralization process. However, Au is precipitated at much higher rates in Au-rich than in Cu-rich porphyry deposits. Using Monte Carlo simulations of petrologic processes, I show that whereas Cu-rich porphyries require large amounts of magma and water to be formed, Au-rich porphyries are the result of a better efficiency of Au precipitation.

Formation of the Tocantinzinho 1989 Ma gold-only deposit (Pará State, northern Brazil), based on sulfur, oxygen, hydrogen, and carbon isotope data

The Tocantinzinho gold deposit in the Brazilian Amazon region, approximately 1989 Ma in age, contains 16.7 Mt of ore in syenogranite and monzogranite breccias and microbreccias, most with gold in pyrite, silicates and quartz, and a small part in quartz vein pyrites. Three hydrothermal phases are recognised, H1, H2, and H3, where H1, the first hydrothermal phase, is the oldest. The H1 transitional, late-magmatic hydrothermal phase altered feldspar to sericite and mafic minerals to chlorite. During the H2 hydrothermal phase, pyrite, gold, and minor galena were precipitated, followed by intense sericite alteration. The last hydrothermal phase, H3, was coeval with a large number of andesite intrusions that generated high-pressure fluid-driven cataclasis. This event opened veins, tension gashes, and pull aparts that were filled with carbonate, worm-like chlorite, prismatic quartz, galena, pyrite, and rutile, which likely displaced and concentrated the gold near the carbonate-filled structures.The minerals sulfide δ34S values vary between −4.4 and + 5.6‰, which is compatible with a sulfur magmatic source, and indicate that the sulfur and likely the gold are both magmatic. The δ18OSMOW values of carbonates vary between +7.63 and + 21.23‰ in a narrow range of δ13CPDB values between −3.94 and −1.69‰. Two explanations are possible for these values. If that the carbonate fluids were genetically related to the andesite magmatism, this fluid mixed with meteoric water, and degassing of this mixture occurred in the near-surface to produce dissolved inorganic CO2 with high δ18O value. Alternatively, when fluid mixing occurred, the oxidation state of the environment increased, and the δ13C values of the carbonates remained relatively uniform, whereas the δ34S values of the sulfides varied. The δ18O values of the quartz, sericite, and chlorite of the mineralized and barren rocks are consistent with δ18O values > +8 for the fluids that crystallized the chlorites and δ18O > + 4‰ for those that crystallized the muscovite. High δD values of these fluids, estimated from mineral δD values range between −2 and −25‰, and differentiate them from the fluids that form porphyry-type Cu–Au and orogenic deposits. The isotope data for Tocantinzinho present characteristics of an intrusion-related gold deposit that combines the features of equigranular, intrusion-hosted, mesozonal deposits, such as those hosted in reduced granite, with Au-rich porphyry deposits hosted in oxidized granites.

Sulfur isotope and PGE systematics of metasomatised mantle wedge

At convergent margins fluids liberated from subducting slabs metasomatise the overlying mantle wedge, enriching it in volatiles, incompatible elements and possibly ore-forming metals. Despite the genetic link between this process, the genesis of arc magmas, and formation of porphyry Cu–Au deposits, there is currently little understanding of the behaviour of chalcophile and siderophile elements during subduction-related mantle metasomatism. In this study, we report sulfur isotopic compositions and PGE concentrations of sulfides in a suite of garnet peridotites from the Western Gneiss Region of Norway, sampling mantle wedge from ∼100 to ∼250 km depth. Sulfides hosted in metasomatised samples have δ34S deviated from typical mantle values, ranging between −10.0 and +5.4‰, indicating derivation of sulfur from subducted crust. Sulfides in pervasively metasomatised samples have atypical PGE signatures, with strong enrichment in Os and Ru relative to Ir, whereas channelised fluid flow produced sulfides extremely enriched in Pd, up to 700 times the concentration found in non-metasomatised samples. These signatures are reconcilable with a high oxidation state of the metasomatising agents and demonstrate that subduction can recycle chalcophile and siderophile elements into and within the mantle, along with sulfur. We further show that because the solubility of Os and Ru in fluids is redox sensitive, and Pd is more soluble than the I-PGE, ratios such as Os/Ir, Ru/Ir plotted against Pd/Ir can be used to trace the metasomatic oxidation of mantle samples, mantle-derived magmas and porphyry Cu±Au deposits. This geochemical insight is used to show that Au-rich porphyry Cu deposits are derived from more oxidised mantle wedge than Au-poor porphyry deposits.

Solubility of gold in oxidized, sulfur-bearing fluids at 500–850 °C and 200–230 MPa: A synthetic fluid inclusion study

Although Au solubility in magmatic-hydrothermal fluids has been investigated by numerous previous studies, there is a dearth of data on oxidized (log fO2 > FMQ+2.5; FMQ – fayalite-magnetite-quartz buffer), sulfur-bearing fluids such as those that formed porphyry Cu-Au (-Mo) deposits. We performed experiments to constrain the effects of fluid salinity, HCl content, sulfur content, fO2 and temperature on Au solubility in such oxidized, sulfur-bearing fluids. For this purpose, small aliquots of fluids equilibrated with Au metal were trapped at high pressure and temperature in the form of synthetic fluid inclusions in quartz and were subsequently analyzed by LA-ICP-MS. Additionally, Raman spectra were collected from quartz-hosted fluid inclusions at up to 600 °C to help to identify the nature of dissolved gold and sulfur species. Gold solubility was found to be affected most strongly by the HCl content of the fluid, followed by fO2, fluid salinity and temperature. Compared to these factors the sulfur content of the fluid has relatively little influence. At 600 °C and 100 MPa, fluids with geologically realistic HCl contents (∼1.1 wt%) and salinities (7–50 wt% NaClequiv) dissolve ∼1000–3000 ppm Au at oxygen fugacities controlled by the magnetite-hematite buffer. At even more oxidized conditions (three log units above the hematite-magnetite fO2 buffer), HCl-, NaCl- and H2SO4-rich fluids can dissolve up to 5 wt% Au at 800 °C and 200 MPa. The observed Au solubility trends are controlled by HCl0 species in the Na-H-Cl-SO4 fluid and are quantitatively reproduced by existing thermodynamic data for Au-Cl complexes. In all experiments, AuCl0 and AuCl2− species are predicted to occur in comparable although variable concentrations, and account for more than 95% of Au solutes. Natural, high-temperature (>500 °C) brine inclusions from porphyry Cu-Au (-Mo) deposits contain significantly less Au than gold-saturated brines that were synthesized experimentally, implying that the natural brines were Au-undersaturated. Consequently, gold grades in Au-rich porphyries were not controlled by the precipitation of native Au, but rather by factors that caused the precipitation of Au-bearing hydrothermal sulfides such as bornite.

Melt recharge, f O2-T conditions, and metal fertility of felsic magmas: zircon trace element chemistry of Cu-Au porphyries in the Sanjiang orogenic belt, southwest China

The magmatic hydrothermal Pulang Cu deposit (Triassic) and the Beiya Au-Cu deposits (Eocene) are located in the Sanjiang copper porphyry belt, southwest China. Zircon chemistry was used to constrain the magmatic evolution and oxidation state of the porphyries. The results show that porphyries of the Beiya district formed from an early oxidized melt and a later relatively reduced and more evolved magma, whereas Pulang experienced a normal Cu porphyry evolutionary trend. The Pulang porphyries crystallized from more oxidized magma (∆FMQ + 2.9–4.6, average = 4.0 ± 1.0, n = 3) with an average temperature of 709 ± 6 °C compared to the Beiya porphyries (∆FMQ + 0.6–3.5, average = 1.9 ± 1.3, n = 5) with a mean magmatic temperature of 780 ± 22 °C. These data, combined with data from other Cu- and Au-rich porphyries in the Sanjiang belt (i.e., Machangjing Cu, Yao’an Au), are consistent with previous experimental work showing that elevated Cu and Au solubilities in magma require oxidizing conditions. A compilation of existing geochemical data for magmatic zircons from fertile and barren porphyry systems worldwide establishes an optimal diagnostic interval on CeIV/CeIII-TTi-in-zircon and (Eu/Eu*)N plots for generating magmatic hydrothermal Cu-Au deposits.

Recycling of metal-fertilized lower continental crust: Origin of non-arc Au-rich porphyry deposits at cratonic edges

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383–445 ppm) and Au (7–12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6–16 ppb) with higher Au/Cu ratios (2 × 10 −4 to 8 × 10 −4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au–fertilized low-crustal cumulates at 40–30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits.

The Nuevo Chaquiro Cu-Au-(Mo) Porphyry Deposit, Middle Cauca Belt, Colombia: Geology, Alteration, Mineralization

Nuevo Chaquiro is a newly discovered blind porphyry Cu-Au-(Mo) deposit located in the Middle Cauca belt, Colombia, approximately 60 km south-southeast of Medellin. The most recent resource estimate was stated as 566 M tonnes (t) averaging 0.64% Cu, 0.31 g/t Au, 4.34 g/t Ag, and 127 ppm Mo. The geology of Nuevo Chaquiro consists of a Miocene volcanic sequence of andesitic tuffs, agglomerates, and flows (Combia Formation) intruded by small stocks and dikes of diorite and quartz diorite also of Miocene age. Sparsely occurring outcrops at Nuevo Chaquiro consist of a 1.2 × 0.8 km zone of pervasive sericitic alteration with locally well developed quartz-Fe oxide stockworks. The sericitic alteration grades downward into sericite-chlorite alteration and then predominantly potassic (biotite-magnetite) alteration at depths of 250 to 400 m. Calcic-potassic alteration, containing actinolite and epidote, occurs at greater depths. A high-grade sheeted vein complex occurs immediately adjacent to the upper contact of an Early quartz diorite intrusion. Quartz veinlet density in this area reaches up to 80% quartz by volume, accompanied by up to 5 vol % chalcopyrite. Later Intermineral porphyries created a second orebody slightly to the west but contiguous with the Early orebody. Here Cu-Au-Mo mineralization in an inverted bowl shape is associated with potassic alteration that manifests itself as secondary flooding of the tuff wall rock matrix by hydrothermal biotite, accompanied by disseminations and hair veinlets of chalcopyrite, magnetite, pyrite, and molybdenite. The porphyry system is also accompanied by intermediate-sulfidation carbonate–quartz-base metal veins, typically with sericitic alteration selvages, which occur on the upper western flank of the deposit. At Nuevo Chaquiro, multiple intrusions of Early and Intermineral porphyry contain calcic-potassic and potassic alteration, with a later overprint by sericite-chlorite alteration centered on intrusion apexes. The amount of sericite-chlorite overprinting varies with each intrusion. Collectively, these intrusions are all overprinted by a single, later sercitic (phyllic) alteration event that is superimposed as a broad cap upon the entire system. There is a consistent paragenesis for the alteration/mineralization associated with each intrusion, suggesting a relatively stable underlying shallow magma chamber that fluxed geochemically similar pulses of magmatic/hydrothermal fluid with each porphyry. In many aspects, Nuevo Chaquiro is a “typical” Au-rich porphyry copper system. However, the contrast between the mineralization styles between the Early and Intermineral orebodies as well the timing differences in copper deposition suggest that differences in porphyry mineralization styles also occur at Nuevo Chaquiro.

Fluid Inclusion Studies on Quartz Veinlets at the Ali Javad Porphyry Copper (Gold) Deposit, Arasbaran, Northwestern Iran

Ali Javad porphyry copper-gold deposit is located in Arasbaran porphyry copper belt at northwestern Iran, some 20 km east of Sungun Mine. Porphyry mineralization at the Ali Javad deposit occurred in post-Oligocene quartz monzonite bodies which intruded in the Eocene volcanic rocks. Mineralization occurred as veins, veinlets and dissemination both as hypogene and supergene type. Several types of veinlets were distinguished during the study of the deposit. Fluid inclusion studies on fluids trapped in quartz which were taken from drill core samples indicated a wide range of homogenization temperature in the veinlets from 138&#176C to 565&#176C which their salinity demonstrated 33 - 61 wt% NaCl equivalent. Mineralizing fluids density at the deposit was 0.8 - 1.2 g/cm3. Fluid inclusion studies suggested that Ali Javad deposit is an Au-rich porphyry copper deposit; its fluid inclusion features were comparable with other porphyry deposits.

Geochemistry and fluid characteristics of the Dalli porphyry Cu–Au deposit, Central Iran

The Miocene Dalli porphyry Cu–Au deposit in the central part of Urumieh–Dokhtar magmatic arc is the first reported Au-rich porphyry Cu deposit in the Zagros orogenic belt. The Cu–Au mineralization is mainly hosted in diorite and quartz diorite intrusions, presenting as numerous veinlets in the altered wall rocks, with potassic, phyllic, and propylitic alteration developed. Based on the mineral assemblages and crosscutting relations of veinlets, hydrothermal mineralization–alteration occurred in at least three stages, characterized by veinlets of (1) Qtz+Kfs+Mag±Ccp, (2) Qtz+Py+Ccp±Bn±Cv±Cc and, (3) Qtz+Chl+Bt. The ore-bearing intrusions exhibit typical geochemical characteristics of subduction zone magmas, including LREE fractionated pattern, strong enrichment in LILE (Cs, Rb, Ba, Pb, and U), and depletion of HFSE, with marked negative Ti and Nb anomalies. The adakite-like ore-hosting porphyry intrusions are characterized by a systematic gradual decreasing and increasing of Y and Eu/Eu∗ with increasing SiO2 content, respectively. Moreover, they exhibit a significant increasing trend of Sr/Y with decreasing of Y, which indicates progressive hornblende fractionation and suppression of plagioclase fractionation during the evolution toward high water content of parental magma. A relatively flat HREE pattern with low Dyn/Ybn and Nb/Ta values may represent that amphibole played a more important role than garnet in the generation of the adakitic melts in the thickened lower crust. Based on the phase assemblages confirmed by detailed laser Raman spectroscopy analyses and proportion of solid, liquid, and gaseous components, five types of fluid inclusions were recognized, which are categorized as; (1) liquid-rich two phase (liquidH2O+vaporH2O) (IIA), (2) vapor-rich two phase (vaporH2O/CO2+liquidH2O) (IIB), (3) high saline simple fluids (IIIA; liquidH2O+vaporH2O+Hl), (4) high saline opaque mineral-bearing fluids (IIIB; liquidH2O+vaporH2O+Hl+Hem+Ccp+Py) and (5) multi-phase type (IIIAB; liquidH2O+vaporH2O+Hl+Anh+Hem+Mag+Ccp). In early stage veins, the homogenization temperature of multiphase inclusions as high as 620°C, with corresponding salinities of up to 75wt.% NaCl equivalent represent the initial ore-forming fluids. From early to late stage veins, the gradual decrease of homogenization temperature of saline inclusions (IIIAB, IIIA, and IIIB) from 620 to 340°C (corresponding salinities of 75–35wt.% NaCl equivalent) may reflect fluid boiling and mixing of the early magmatic fluids with circulating groundwater. The common association of hematite and anhydrite daughter phases with the most primitive inclusions (IIIAB) in early-stage veins and the lack of CO2-bearing inclusions in the middle to late stage veins reveal CO2 content and oxygen fugacity of the fluids was significantly decreased from early to main stage of sulfide mineralization (second generation of veins). It seems that magnetite crystallization and CO2-escape have a decisive role in oversaturation of S2− and subsequent rapid and large-scale precipitation of sulfides in second generation of veins at the Dalli deposit.

Formation of Au-polymetallic ore deposits in alkaline porphyries at Beiya, Yunnan, Southwest China

The Beiya gold deposit is located in the central part of the Jinshajiang–Honghe strike–slip belt, at the junction zone of the Tethys-Himalaya orogenic belt and the Yangtze plate in SW China. This large-scale (125.6millionmetrictonnes with a mean grade of 2.42g/t) Au-rich polymetallic deposit is related to alkaline porphyry intrusions. Previous studies show that the Beiya alkaline porphyry intrusions are the fractionation product of a mantle-derived magma emplaced in an extension environment at the post-collision stage of the India–Eurasian plate collision; the Beiya deposit is considered to be skarn-related. Based on detailed field studies and previous work, we propose that the Beiya porphyry and associated Au-rich polymetallic ores were formed by the emplacement of magmas within the Jinshajiang–Honghe strike–slip fault during the late stage of the India–Eurasian plate collision at 45–25Ma.At Beiya, the mineralized zones in the Cu–Au-rich porphyries are surrounded by Au–Cu–Fe skarns and Au (Cu) veins. Pb–Zn–Ag-rich mineralization was derived from the inner porphyry, and is widely developed outside the central alkaline-rich porphyry.The sulfur isotope signature of the sulfide mineralization in the Beiya and Qinhe deposits is −2.40‰–4.50‰ and 1.25–2.75‰, respectively. These values are close to 0‰, indicating that the sulfur may be mantle-derived. The δ18O compositions of the ore-forming fluids responsible for the formation of calcites at Qinhe are 8.10‰–9.61‰, which is lower than that of Beiya (δ18O=10.5‰) where the ores contain a larger contribution of oxygen from the mantle. The Beiya porphyry magmas provided fluids and the heat that drove the transport of the metals to the site of deposition.Alkaline porphyries are widely distributed throughout the Jinshajiang–Honghe strike–slip fault belt, and they are potential hosts to future discoveries of Beiya-style mineralization.

Marine Volcanosedimentary Basins Hosting Porphyry Au-Cu Deposits, Cadia Valley, New South Wales, Australia

The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Tectonic Controls on Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au Ag-Pt-Pd-Mo) Within the Canadian Cordillera

Paired belts of alkalic and calc-alkalic porphyry deposits are genetically linked to early Mesozoic volcanic arcs preserved as the Quesnel and Stikine terranes in western Canada. These parallel arc terranes extend for 2,000 km along the axis of the Canadian Cordillera. They are joined at their northern ends, but are otherwise separated by relics of Tethyan ocean basin and oceanic arc rocks collectively referred to as the Cache Creek terrane. Porphyry Cu ± Au-Mo-Ag deposits are concentrated within the Stikine-Quesnel arc terranes, with most of their economic metal endowment a consequence of a 15-m.y. mineralizing epoch. A particularly prolific period within the mineralizing epoch is a 6-m.y. pulse centered on 205 Ma when more than 90% of the known copper endowment was acquired. Distinct trends of Cu-Au ± Ag-Mo mineralization within both arc terranes coincide in time and space with events that are attributed to effects of slab subduction. Stratigraphy within both Stikine and Quesnel arc terranes is equivalent, with greater variation along the arcs than between them. Magmatism within both arcs began in the Late Devonian, following initiation of subduction near eastern Panthalassa and formation of the back-arc Slide Mountain ocean basin margin adjacent to Laurentia. Both terranes record further arc development in the lower Carboniferous and cessation in the Permian. Between the Late Permian and Middle Triassic, both arcs foundered, and then recorded a cryptic Early Triassic deformational event, coincident with closing of the Slide Mountain ocean. Permo-Triassic subduction and arc magmatism are recorded in the Sitlika-Kutcho-Venables arc, but this occurred far to the west, within the Tethyan realm. Early in the Late Triassic (early Carnian), both the Stikine and Quesnel arcs reignited. Later, near the end of the Triassic (late Norian), the arcs were uplifted, picrites were erupted locally, and magmatism was dominantly strongly alkaline. Monzonitic plutons hosting alkalic Cu porphyry deposits were emplaced along the arcs, followed by a widespread erosional hiatus that extends throughout much of the latest Triassic (Rhaetian). Late Triassic events culminating in generation of Cu porphyry deposits are attributed to a model wherein delivery of the Permo-Triassic Sitlika-Kutcho-Venables arc to the Stikine-Quesnel arc trench led to stalled subduction and arc-parallel tearing of the slab. Ingress of hot subslab mantle into the tear resulted in high-degree partial melts, as recorded by picrite within the arc circa 210 Ma. Widening of the tear and decay of the initial thermal spike raised temperatures across broad regions of the metasomatized mantle wedge, with low-degree partial melts generated from the most hydrated, metal-enriched portions circa 203 Ma. These melts produced the well-defined belt of alkalic Cu-Au-Ag porphyries. Parts of the slab tear may have nucleated near the forearc to generate Mo-rich Cu porphyries, such as the Gibraltar deposit, which partly cuts the accreted Cache Creek terrane. Metal-rich hydrous melts may have traveled along preexisting intrusive pathways to invade batholithic bodies along the arc axis, such as at Highland Valley, where 204 Ma mineralization is hosted within a larger ~210 Ma plutonic complex. Migration of the locus of mineralized porphyry intrusions toward the backarc may track the leading edge of the slab gap, with the formation of Cu-Au and younger Au-rich porphyries until ~195 Ma. Late alkalic porphyry formation, following emplacement of the Quesnel arc onto the ancestral North American margin at ~186 Ma, may have been produced through cannibalism of mineralized intrusions emplaced at deeper crustal levels during the ~203 Ma Cu porphyry event. Two calc-alkalic porphyry deposits that predate Sitlika-Kutcho-Venables arc collision—Fin and Schaft Creek (~220 Ma)—are temporally related to arc uplift and erosion across the northern Stikine arc, but little is known about the extent and cause of this event and how it relates to porphyry Cu genesis. Slab tears may play a dominant role in the formation of porphyry deposits globally. Previous workers have linked episodes of porphyry formation with subduction of ocean plateaus, changes in subduction polarity, or termination of subduction. In each case, slab tears are the likely underlying cause of porphyry formation. We believe them to be fundamental to mineralization in the early Mesozoic arc terranes of Stikine and Quesnel.

The relation between Cu/Au ratio and formation depth of porphyry-style Cu–Au ± Mo deposits

Constraints on gold and copper ore grades in porphyry-style Cu–Au ± Mo deposits are re-examined, with particular emphasis on published fluid pressure and formation depth as indicated by fluid inclusion data and geological reconstruction. Defining an arbitrary subdivision at a molar Cu/Au ratio of 4.0 × 104, copper–gold deposits have a shallower average depth of formation (2.1 km) compared with the average depth of copper–molybdenum deposits (3.7 km), based on assumed lithostatic fluid pressure from microthermometry. The correlation of Cu/Au ratio with depth is primarily influenced by the variations of total Au grade. Despite local mineralogical controls within some ore deposits, the overall Cu/Au ratio of the deposits does not show a significant correlation with the predominant type of Cu–Fe sulfide, i.e., chalcopyrite or bornite. Primary magma source probably contributes to metal endowment on the province scale and in some individual deposits, but does not explain the broad correlation of metal ratios with the pressure of ore formation. By comparison with published experimental and fluid analytical data, the observed correlation of the Cu/Au ratio with fluid pressure can be explained by dominant transport of Cu and Au in a buoyant S-rich vapor, coexisting with minor brine in two-phase magmatic hydrothermal systems. At relatively shallow depth (approximately <3 km), the solubility of both metals decreases rapidly with decreasing density of the ascending vapor plume, forcing both Cu and Au to be coprecipitated. In contrast, magmatic vapor cooling at deeper levels (approximately >3 km) and greater confining pressure is likely to precipitate copper ± molybdenum only, while sulfur-complexed gold remains dissolved in the relatively dense vapor. Upon cooling, this vapor may ultimately contract to a low-salinity epithermal liquid, which can contribute to the formation of epithermal gold deposits several kilometers above the Au-poor porphyry Cu–(Mo) deposit. These findings and interpretations imply that petrographic inspection of fluid inclusion density may be used as an exploration indicator. Low-pressure brine + vapor systems are favorable for coprecipitation of both metals, leading to Au-rich porphyry–copper–gold deposits. Epithermal gold deposits may be associated with such shallow systems, but are likely to derive their ore-forming components from a deeper source, which may include a deeply hidden porphyry–copper ± molybdenum deposit. Exposed high-pressure brine + vapor systems, or stockwork veins containing a single type of intermediate-density inclusions, are more likely to be prospective for porphyry–copper ± molybdenum deposits.

Gold grain morphology and composition as an exploration tool: application to gold exploration in covered areas

The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; &gt;300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.

THE FARALLÓN NEGRO GROUP, NORTHWEST ARGENTINA: Magmatic, Hydrothermal and Tectonic Evolution and Implications for Cu-Au Metallogeny in the Andean Back-arc

Research Article| July 01, 1998 THE FARALLÓN NEGRO GROUP, NORTHWEST ARGENTINA: Magmatic, Hydrothermal and Tectonic Evolution and Implications for Cu-Au Metallogeny in the Andean Back-arc ANNE M. SASSO; ANNE M. SASSO (SEG 1991) DEPARTMENT OF GEOLOGICAL SCIENCES, QUEEN’S UNIVERSITY, KINGSTON, ONTARIO, CANADA K7L 3N6 EMAIL:amsasso@aol.com Search for other works by this author on: GSW Google Scholar ALAN H. CLARK ALAN H. CLARK DEPARTMENT OF GEOLOGICAL SCIENCES, QUEEN’S UNIVERSITY, KINGSTON, ONTARIO, CANADA K7L 3N6 Search for other works by this author on: GSW Google Scholar SEG Discovery (1998) (34): 1–18. https://doi.org/10.5382/SEGnews.1998-34.fea Article history first online: 15 Sep 2021 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation ANNE M. SASSO, ALAN H. CLARK; THE FARALLÓN NEGRO GROUP, NORTHWEST ARGENTINA: Magmatic, Hydrothermal and Tectonic Evolution and Implications for Cu-Au Metallogeny in the Andean Back-arc. SEG Discovery 1998;; (34): 1–18. doi: https://doi.org/10.5382/SEGnews.1998-34.fea Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietySEG Discovery Search Advanced Search Argentina has rapidly emerged as a major focus of international exploration activity. With the confirmation of the giant status (sensuClark, 1993) of the Bajo de la Alumbrera porphyry Cu-Au deposit (Guilbert, 1995), few areas have generated greater interest than the Fara116n Negro region of Catamarca Province (Fig. 1). Bajo de la Alumbrera (MIM/North/Rio Algom), the largest Au-rich (0.4 g/t) porphyry copper deposit recognized in the Central Andean orogen, with delimited reserves of 780 M tonnes containing 0.52% Cu and 0.67 g/t Au at a 0.22% Cu cut-off (Mining Journal Ltd., 1997... First Page Preview Close Modal You do not currently have access to this article.

Chemical mobility of gold in the porphyry-epithermal environment

Using recently published experimental data, we have calculated the solubility of gold for simplified magmatic fluids that cool between 500 degrees and 300 degrees C. The starting fluid has the following characteristics: P = 1 kbar, Sigma Cl = 2.0 m, Sigma KCl/Sigma NaCl = 0.25, pH fixed by muscovite + K feldspar + quartz, f (sub O 2 ) fixed by SO 2 /H 2 S, and a (sub H 2 S) fixed by magnetite + pyrite. Parallel calculations were performed asstuning no drop in pressure during cooling (isobaric model) or an instantaneous drop in pressure to 500 bars, resulting in separation of a dense brine and a low-salinity vapor (boiling model). The isobaric model applies to magmas emplaeed at hypozonal or mesozonal depths, whereas the boiling model is more appropriate for shallow porphyry deposits.In the isobaric model, gold solubility is initially dominated by AuCl 2 (super -) at 500 degrees C, 1 kbar. If H 2 S levels are high (pyrite stable), the dominant complex shifts to Au(HS) 2 (super -) upon cooling below approximately 450 degrees C, and solubilities remain elevated (> 100 ppb) over the entire temperature range. If H 2 S levels are low (magnetite stable), gold solubility decreases steadily to 300 degrees C, with AuCl 2 (super -) the dominant complex throughout. Thus, gold dissolved in H 2 S-rich fluids will tend to be carried away from the parent magma, whereas gold in H 2 S-poor fluids will tend to precipitate closer to the source. At 500 degrees C, gold solubility as AuCl 2 (super -) is highest for fluids that are oxidized (SO 2 /H 2 S > 1), acidic. highly saline, and potassium rich. Gold may precipitate in response to a number of mechanisms, including cooling, pH increase, and dilution.Magmatic fluids that evolve from shallow porphyry, bodies are apt to boil shortly after leaving the melt, at which point most of the dissolved gold will partition along with chloride into the brine phase. This metal-rich fluid, because of its high density, will tend to sink or reflux near the parent intrusion, possibly forming an Au-rich porphyry Cu deposit. Mass balance calculations suggest that magmatie brines will initially be undersaturated with respect to metallic gold, although the metal may still precipitate as Au-rich copper sulfide minerals (iss, bornite). During boiling, most of the H 2 O and H 2 S will partition into the coexisting vapor phase. As this vapor cools, it may recondense into a low-salinity, H 2 S-rich xvater of mixed magmatie-meteoric heritage that has a high potential for dissolving and remobilizing significant quantities of gold as Au(HS) 2 (super -) . Migration of this fluid to shallower levels may eventually form epithermal deposits of low- or high-sulfidation affinity, depending on the pH-buffering capacity of the wall rocks, and the extent of direct magmatie involvement. Lack of contact with retrograde, H 2 S-rich magmatic-meteoric waters may be a prerequisite for the preservation of early Au-rich porphyry-style mineralization and may also explain the observed association between gold and hypogene iron oxide alteration in many porphyry deposits.Whether or not there is a direct temporal link betxveen ore-forming processes in the porphyry and epithermal regimes, an intrusive event may be important as a means of introducing a large quantity of low-grade gold which is then available for later remobilization and concentration by circulating fluids of nonmagmatic origin. Moreover, an early porphyry event may cause widespread sulfidation of surrounding rocks. Later fluids of meteoric origin circulating through this pyrite-rich wall rock will have H 2 S concentrations that remain elevated during cooling and ascent, increasing the chances of forming a large, high-grade epithermal gold deposit.